Flyback converters are well known in the art and FIG. 1 of the accompanying drawings shows the main components of a basic flyback converter 10. The converter 10 comprises an input circuit 11 coupled to an output circuit 12 through an energy-storing transformer 13 that has primary and secondary windings 14 and 15 respectively. The input circuit 11 comprises a switching device 16 (shown as a MOSFET in FIG. 1) connected in series with the transformer primary winding 14, and a control block 17 for controlling the cyclic turning on and off of the switching device 16. The output circuit 12 comprises a diode 18 connected in series with the transformer secondary winding 15, and a reservoir capacitor 19 connected in parallel with the series combination of the secondary winding 15 and diode 18 and across which the converter output is produced. A load 20 is connected across the converter output.
The basic operation of the Figure converter is fairly simple and will now be briefly described with reference to the diagrammatic time plots of FIG. 2. The MOSFET switching device 16 is cyclically turned on by a waveform V.sub.gs applied to its gate, the MOSFET being on (see waveform V.sub.ds, the voltage across the MOSFET) when the gate voltage waveform V.sub.gs is high. When the MOSFET is on, current I.sub.d builds at a steady rate through the transformer primary winding 14 and energy is stored in the flux in the transformer core. During this phase, no current flows in the secondary winding as the diode 18 is reverse biased, and the load 20 is supplied with current from the reservoir capacitor 19.
When the gate voltage V.sub.gs goes low, the MOSFET turns off and the magnetic flux in the transformer progressively collapses driving a current I.sub.rect through the secondary winding 15 and diode 18 towards the load 20. In the present example, the flux in the transformer core does not fall to zero prior to the MOSFET turning on again to restart the cycle of operation; as a result, the current I.sub.rect also does not reach zero before the cycle restarts. This mode of operation is known as the continuous mode. It is also possible for the converter to operate in a discontinous mode in which the transformer is fully discharged before the MOSFET is turned on again in which case the current I.sub.rect will fall to zero earlier than the turn on of the MOSFET.
The output voltage of the converter 10 is regulated by the feedback of the output voltage to the control block 17. Generally, the output circuit will be electrically isolated from the input circuit so that the feedback path will normally include an isolating element (this is signified in FIG. 1 by using a dashed oval to depict the feedback loop as only indirectly connected to the output circuit). Typically, the switching frequency of the switching device 16 is fixed and the control block 17 controls the duty cycle of the switching device (i.e. effects a pulse width modulation control) to keep the output voltage constant.
In the waveform of the voltage V.sub.ds across the MOSFET, a voltage spike can be seen immediately following turn off of the MOSFET, this spike being superimposed in the voltage step present as the supply voltage appears across the MOSFET. This voltage spike is due to the dissipation of the energy stored in the primary leakage inductance during the immediately preceding period when the MOSFET was-on and the transformer was charging. Since the energy in the leakage inductance cannot be discharged on the secondary side of the transformer, it will be dissipated in the transformer or MOSFET unless a dummy load ("snubber") is provided on the primary side; the provision of snubber circuits is normal practice and is shown in FIG. 1 by the dotted components.
For a transformer in a flyback converter for a mains-powered, low voltage power supply unit (for example, a 5v supply for electronic equipment), the need to comply with a variety of safety regulations means that the leakage inductance can be up to one fifth of the main inductance. The efficiency of the power supply in this case is thus immediately limited to 80% due just to this phenomenon; the practical efficiency will then be around 60% for low line operations.
It is an object of the present invention to provide a flyback converter in which the energy loss due to primary leakage inductance is reduced.